Make SystemTime mockable via object #5223

Job	Run time
test on solaris	10m 50s
test on freebsd	4m 15s
test on netbsd	5m 18s
audit	26s
features (ubuntu-latest)	1h 22m 57s
lint	1m 11s
features (macos-latest)	23m 20s
msrv	1m 3s
features (windows-latest)	50m 14s
test (ubuntu-latest, stable)	1m 34s
test (ubuntu-latest, beta)	1m 12s
test-android (x86_64-linux-android, x86_64, 26)	2m 45s
test (macos-latest, stable)	1m 23s
test-android (i686-linux-android, x86, 26)	3m 0s
test (windows-latest, stable)	4m 26s
test-aws-lc-rs	1m 37s
test wasm32-unknown-unknown	48s
test on illumos	6m 46s
	3h 23m 5s

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